1. Field of the Invention
This invention relates to a scanning apparatus using a plurality of light sources such as semiconductor lasers or light-emitting diodes as a light source unit.
2. Description of the Prior Art
When an attempt is made to make the pitch of scanning lines dense by using a so-called semiconductor laser array having a plurality of light output portions arranged side by side as shown in Japanese Laid-open Patent Application No. 158251/1979, the direction of the semiconductor laser array must be obliquely inclined instead of being made orthogonal to the scanning lines. Each light beam from such a semiconductor laser array is emitted with a light ray parallel to the normal to the end surface of the array as the central ray.
Referring to FIG. 1 of the accompanying drawings which shows an example of the plural-beam scanning apparatus according to the prior art, central rays ha and hb are emitted from a plurality of light output portions 1a and 1b provided in a semiconductor laser device 1, in parallelism to the optical axis g of a condenser lens 2. These central rays pass through the focus F of the condenser lens 2 and through a cylindrical lens 3 to the deflecting-reflecting surface 4a of a deflector 4. At this time, the central rays ha and hb from the light output portions 1a and 1b are reflected on the deflecting-reflecting surface 4a at positions spaced apart from each other in the direction of deflection of the light beam by the deflector.
The light beams emitted from the light output portions 1a and 1b and passed through the condenser lens 2 are formed as line images near the deflecting-reflecting surface 4a by the cylindrical lens 3.
The light beams reflected by the deflecting-reflecting surface 4a are imaged on the surface of a medium 6 to be scanned such as a photosensitive medium by an anamorphic scanning lens system 5.
FIG. 2 of the accompanying drawings shows the manner of imaging in a plane orthogonal to the plane of the drawing sheet of FIG. 1. In FIG. 2, use is made of an anamorphic scanning lens for making a point near the deflecting-reflecting surface 4a and a point on the surface of the photosensitive medium conjugate.
The anamorphic scanning lens 5 comprises, for example, a spherical lens 5a and a toric lens 5b.
The central rays ha and hb emitted from the light output portions 1a and 1b of the semiconductor laser device enter the deflecting-reflecting surface 4a at the positions thereon spaced apart from each other, as previously described. That is, as regards the center positions of the line images formed by the cylindrical lens 3, the center position of only the line image corresponding to any one of the plurality of light output portions can be set up at a desired position, but the center positions of the other line images are spaced apart from the desired position. The optical path indicated by broken lines in FIG. 2 shows the manner of imaging in this case, and the center position P of the deviated line image is deviated from the desired position and is not imaged on the surface of the medium to be scanned by the lens system 5, but is imaged at a point P' spaced apart from the surface of the medium to be scanned, with a result that so-called defocus is created. When the deflecting mirror 4 is rotated, the amount of defocus becomes remarkably great and thus, the spot imaged on the surface of the medium to be scanned becomes large.